Molding in a semiconductor lead frame package is performed to protect a semiconductor integrated circuit die attached to a metal lead frame substrate in a conventional packaging process. Molding is carried out in a mold press or molding machine. Typically, the molding machine includes a mold chase with two mold halves, which sandwich the lead frame with the semiconductor dice positioned within cavities formed in the mold halves. Encapsulation material, such as epoxy molding compound (“EMC”), is introduced into the cavity of the mold, thereby encapsulating the semiconductor dice.
One semiconductor package that has been developed fairly recently is the quad, flat, no-lead package sometimes known as a QFN package. A characteristic of a QFN package is that EMC is molded around a semiconductor die on only one side of the lead frame substrate. The opposite side of the substrate is substantially free of EMC. QFN packages generally fall into one of two categories, either matrix QFN or block QFN.
For the matrix QFN package a single die is attached to each die pad and during molding, each die is located within its own mold cavity. Hence, each die is individually encapsulated on the lead frame. After molding, the lead frame is singulated by cutting the metal of the lead frame between each encapsulated die.
As for the block QFN package, the lead frame has separate blocks of die pads to which dice are attached, and each block has a square array of die pads. During molding, all the dice on the die pads in the same block are located in the same mold cavity. Hence, all the dies in the same block are encapsulated in the same block of molding and singulation is performed by cutting through the lead frame and the molding material between the dies in a block.
In both matrix and block QFN packages the overall size of the package is close to the size of the semiconductor chip located within the package. Thus, these packages are also referred to as chip-scale packages. In addition, QFN packages are lead frame based packages and have an exposed die paddle. Hence, QFN packages enable good electrical performance and thermal characteristics, as the exposed die paddle can be used as a ground plane and a heat sink.
However, one problem with QFN packages and other semiconductor packages which are molded on only one side of the substrate, is the problem of epoxy flash seeping onto the other side of the lead frame during the molding operation.
One method that has been developed to avoid the problem of epoxy flash seepage is to paste an adhesive tape to a surface of the lead frame that is to be free of epoxy. This method is described in U.S. Patent Publication No. 2003/0082854A1 entitled, “Lead Frame, Method of Manufacturing the Same, and Method of Manufacturing a Semiconductor Device Using the Same”. Mass molding of a lead frame is performed to a lead frame that has an adhesive tape attached to an entire back surface of the lead frame, so as to prevent a leakage of molding resin from the back surface of the lead frame. Generally, the attachment of the adhesive tape to the back surface of the lead frame (taping) is performed prior to the package assembly process.
A problem with this approach is that lead frame warpage may occur during molding due to a difference in the coefficient of thermal expansion between a material constituting the lead frame and the material constituting the adhesive tape. Furthermore, the taping process is relatively expensive because the adhesive tape has to be applied individually to each lead frame prior to the package assembly process. The material cost of the adhesive tape itself (which is not reusable) is also significant. In fact, the disclosure in the aforesaid invention suggests sectioning the adhesive tape into smaller pieces and attaching them to the lead frame, which would further increase the fabrication steps and costs in preparing each piece of lead frame for molding. Furthermore, means must be provided to remove the adhesive tape after molding, and some adhesivity may even remain on the lead frame surface that may adversely affect handling of the lead frame by downstream processes after molding.
An alternative to attaching an adhesive tape to a back surface of each individual lead frame is disclosed in U.S. Pat. No. 6,674,165 entitled, “Mold for a Semiconductor Chip”. The said patent provides a mold for a semiconductor chip comprising two mold halves, wherein one mold half comprises sealing means adapted to exert a sealing pressure between a surface of the mold and a surface of a substrate located in the mold during a molding operation. It was found that with a high clamping pressure, a mold surface coated with the sealing means can be effective to prevent the molding compound from seeping into an unmolded surface of the substrate.
The sealing means may comprise an elastically deformable coating which is preferably heat resistant and may be, for example, a silicone or fluorine based rubber coating. It is typically laminated onto the surface of the lower mold half. Nevertheless, it has been found that if molding is performed on the sealing means and there is direct contact of epoxy molding compound (“EMC”) on the rubber coating, the EMC will tend to abrade the rubber coating and shorten its useful life span. Once the rubber coating is abraded, it would be necessary to change the coating. That could be expensive and time-consuming.
It would be advantageous to prolong the life of the rubber sealing means without reducing the effectiveness of the sealing means.